If you painstakingly took the time to rotate brushes to be perfectly square, then sure you won't get holes or errors. If you didn't though, and one slightly off brush runs up against another slightly off-brush, it happens. Grid snap makes life easier, especially for UED novices.
Considering every tutorial I've read on UED mentions grid snap and possible problems that can come from turning it off, and my own personal experiences....urban legend it ain't. I think I was at about 600 brushes in my map when the errors started occurring.